An oxidative degradation phenomenon of a cable insulating coating material is explained typically in “Degradation of Polymer Materials”, Corona Publishing Co., Ltd., pages 20-22 (1958) and “Practical Encyclopedia of Plastics”, Industrial Research Center of Japan, Inc., pages 800-807 (1993).
In plastics, when a molecular bond of an alkyl group is cleaved by the action of energy typically of heat or light, a radical (R.) is generated. In an environment where oxygen is present, the radical combines with oxygen to form a peroxy radical (ROO.).R.+O2→ROO.  (Formula 1)
The peroxy radical is highly reactive, withdraws hydrogen from another molecule, and is converted into a peroxide (ROOH) and a radical (R.).ROO.+RH→ROOH+R.  (Formula 2)
The newly generated radical (R.) forms another new peroxy radical in the presence of oxygen according to Formula 1. Independently, the peroxide (ROOH) is also unstable and decomposes to form a peroxy radical (ROO.), an oxy radical (RO.), and/or a radical (R.) consequently.ROOH→RO.+.OH  (Formula 3)2ROOH→ROO.+RO.+H2O  (Formula 4)RO.+RH→ROH+R.  (Formula 5)
As is described above, initially generated one radical (R.) proliferatively forms, via a peroxy radical (ROO.), a multiplicity of new radicals, thus oxidative degradative reactions proceed in a chain. For this reason, cable insulating coating materials requiring a long-term life are added with a radical scavenger as a primary antioxidant so as to suppress oxidative degradative reactions as chain reactions.
Phenolic antioxidants and aromatic amine antioxidants are known as radical scavengers. The phenolic antioxidants prevent the reaction of Formula 2 and thereby prevent the generation of a new radical (R.), in which their phenol group (—OH) gives hydrogen to the peroxy radical and the phenol group itself changes into a stable phenoxy radical (—O.). Such phenol groups are consumed through radical scavenging, but radical chain reactions are suppressed and oxidative degradation is inhibited from proceeding as long as phenol group(s) remains. However, as phenol groups are consumed and depleted, radical scavenging becomes no longer sufficient to suppress radical chain reactions, and this causes oxidative degradation to proceed rapidly.
During proceeding of oxidative degradation, carbonyl groups (C═O) typically of aldehydes, ketones, and carboxylic acids are formed, and, in addition, crosslinking and molecular weight reduction due to cleavage of molecular chain occur. These are described in “Degradation of Polymer Materials”, Corona Publishing Co., Ltd., pages 20-22 (1958). Both molecular weight reduction and crosslinking cause the cable insulating coating material to have a lower fracture elongation with respect to tension and thereby cause the cable to reach the end of its life.
For actual evaluation of cable life, tensile tests are widely employed. Some criteria for evaluation or determination of the life have been proposed, while they may vary depending on the type of the cable insulating coating material and the intended use of the cable. For example, Japanese Unexamined Patent Application Publication (JP-A) No. H10-96712 describes a technique on a method for evaluating a cable life in a nuclear power plant. In this technique, the life is defined as the time when a fracture elongation reaches 100% or below. This technique employs a temperature acceleration test for evaluating a long-term life within a short period of time. In this method, lives of samples undergone aging deterioration at two or more different temperatures are plotted as Arrhenius plotting, and a life at an assumed working temperature is determined by extrapolation. According to this technique, the testing duration can be shortened, because degradation is accelerated at higher temperatures, and the sample reaches its life within a short period of time. However, elevation of the testing temperature has a limitation, because degradation reactions and phenomena may change when the testing temperature exceeds a threshold temperature such as melting point or decomposition temperature of a constitutive material, and this causes the slope of the Arrhenius plot to vary.
The fracture elongation in a life test of a cable insulating coating material added with an antioxidant tends to little decrease for the time being and to abruptly decrease toward the end of life. This is because proceeding of oxidative degradation is suppressed while the antioxidant is consumed, but thereafter the antioxidant is exhausted and becomes failing to scavenge radicals sufficiently, and oxidative degradation rapidly proceeds through chain reactions.
As a technique for evaluating degradation of a cable member by a method other than tensile tests, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-346836 discloses a method for determining whether the cable member reaches the end of its life or not based on measurements such as an absorbance ratio measured using a Fourier transform infrared spectrophotometer, an oxidation lag phase measured using a differential scanning calorimeter, and a thermal decomposition starting temperature measured using a thermogravimeter.
In life evaluation of a cable insulating coating material having a long service life, reduction in evaluation time by elevating the testing temperature has a limitation even when a temperature acceleration test is employed, because a straight-line approximation is not established at excessively elevated temperatures, as is described above. Typically, wires (electric wires) for use in nuclear power plants being operated over a long term and playing an important role in the entire society have to have reliability over a longer period of time than wires for general machinery and wires for outside wiring each requiring a not-so-long service life. According to conventional techniques, it takes a long time to perform a life evaluation test on these wires even when the test is a temperature acceleration test.
Such wires and cables in nuclear power plants will be described with reference to FIG. 1. As illustrated in FIG. 1, various control/instrumentation cables 2, power cables 3, and electrical conduits 8 are laid out in a reactor container 1, and these are passed through a container electric penetration assembly 10, are supported by an electrical conduit 5 or a cable tray 4 provided outside of the reactor container, and are connected typically to a controller 7, a central control panel 6, and/or a dose meter 9. These cables are said to measure from about 1000 to about 2000 km in full length. It is considered that a life span necessary for cables in nuclear power plants is about 60 years, but rapid evaluation and prediction of the life of cables is still required, as is described above.
In addition, when life evaluation based on the fracture elongation in a tensile test is adopted to a cable insulating coating material containing an antioxidant, reduction in fracture elongation is not remarkable until the antioxidant is exhausted, and, as a result, evaluation should be continued to the end of the life. Owing also to this, it takes a long time to evaluate the life of the coating material.
It may be also possible to employ, as an index for degradation, not the fracture elongation, but another measurement such as an absorbance ratio measured using a Fourier transform infrared spectrophotometer, an oxidation lag phase measured using a differential scanning calorimeter, or a thermal decomposition starting temperature measured using a thermogravimeter, as in the technique disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2000-346836. However, the technique disclosed in the patent literature uses the measurement as an index for determining whether the tested material reaches the end of its life or not by comparing the measurement with a threshold, and the threshold used as a criterion is determined from a measurement of a sample undergone long-term degradation. The technique is therefore effective for helping the evaluation to have higher precision and for simplifying the test. However, the literature fails to refer to shortening of the evaluation period including the determination of the threshold used as a criterion.
Japanese Unexamined Patent Application Publication (JP-A) No. H10-96712 discloses a technique as a method for diagnosing a degradation level of a cable insulating coating layer. In this technique, light is applied to the insulating coating layer (insulating coating material) to generate sound due to photoacoustic effect; based on the sound, degree of oxidation, degree of hardening, and degree of radiation degradation, for example, of the insulating coating material are evaluated, and thereby the degradation level of the cable insulating coating material is evaluated.
Japanese Unexamined Patent Application Publication (JP-A) No. H10-115601 discloses a method for diagnosing the degradation level of a cable insulating coating material by applying light to the cable insulating coating material, detecting the oscillation of elastic body (elastic waves) propagating through the insulating coating material with a sensor, and analyzing the oscillation.
The degradation mechanism of a crosslinked polyethylene widely used as an cable insulating coating material in nuclear power plants, and the action of an antioxidant will be illustrated with reference to FIG. 2. The crosslinked polyethylene gives a radical (—CH2—CH.) in a radiation environment, and this radical is converted into a peroxy radical (—CH2OO.) and thereby cleaves the principal chain of the polyethylene. A terminal hydroxyl (OH) of the antioxidant scavenges the peroxy radical and thereby suppresses the cleavage of the principal chain of the polyethylene.
An object of the present invention is to provide a technique for estimating, within a short time, a life of a cable insulating coating material having a long service life.